Ultrasonic tissue imaging method and apparatus with doppler velocity and acceleration processing

ABSTRACT

An ultrasound tissue imaging system having an acoustic transducer, B-mode imaging means to produce with said transducer an electronically scanned B-mode image of tissue under examination, Doppler imaging means that accepts and processes large amplitude, low frequency signals to produce with said transducer an electronically scanned acoustic image of moving tissue, and color display means for displaying the B-mode image as a two-dimensional image with echo intensities encoded using a first mapping function and for simultaneously displaying Doppler information from moving tissue as a two-dimensional image using a second and distinct mapping function that is spatially coordinated with and superimposed upon said B-mode image to augment the B-mode image.

BACKGROUND OF THE INVENTION

In B-mode medical ultrasound imaging, the scanner transmits ultrasonicbursts into the body and detects the energy of ultrasonic echoesbackscattered from both stationary and moving tissues in the body.Ultrasonic scanners may also offer two dimensional Doppler flowdetection (also referred to as color Doppler imaging) to detect andimage moving blood. Recently, some investigators have proposed usingcolor Doppler blood flow imaging capabilities to image moving structuresother than blood, for example the moving heart structure.

In B-mode imaging, stationary and moving targets are both imaged. Oftenthese images are degraded by the presence of stationary noise or clutterfrom both electrical sources and acoustic sources. In cardiac ultrasoundapplications, the moving myocardium is of principal interest, and can beobscured by stationary noise detected with B-mode imaging. Color Dopplerimaging methods will remove this stationary clutter but cannot detectmoving myocardium with the required sensitivity and specificity, sinceDoppler processing has been optimized for detecting blood velocities andnot slower moving myocardium. The characteristics of ultrasonic signalsreturning from moving myocardium are different from those signalsreturning from blood. For example, in blood flow detection, theprocessor recognizes echoes from the strong, slow moving heart tissue asclutter and removes them from the image by means of a stationary targetcanceller or a high pass filter or both. Such a high pass filter isillustrated in the system described in U.S. Pat. No. 5,014,710 entitled"Steered Linear Color Doppler Imaging" for the color Doppler path. It isprecisely those strong, slow moving targets that correspond to movingmyocardium, but they, too, are removed from the color Doppler image.Therefore, prior art color Doppler imaging systems have had difficultyimaging slow moving myocardium.

SUMMARY OF THE INVENTION

An ultrasound tissue imaging system having an acoustic transducer,B-mode imaging means to produce with said transducer an electronicallyscanned B-mode image of tissue under examination, Doppler imaging meansthat accepts and processes large amplitude, low frequency signals toproduce with said transducer an electronically scanned acoustic image ofmoving tissue and color display means for displaying the B-mode image asa two-dimensional image with echo intensities encoded using a firstmapping function and for simultaneously displaying Doppler informationfrom moving tissue as a two-dimensional image using a second anddistinct mapping function that is spatially coordinated with andsuperimposed upon said B-mode image to augment the B-mode image.

The present invention images moving tissue with color Doppler imagingmeans that has the new features of removing or bypassing the circuitthat rejects stationary or slow moving targets and of modifying theprocessing circuits so that they can detect large amplitude, lowfrequency signals. The invention provides new color maps that color bothforward and reverse motion in the same way, as well as Dopplergrey-scale maps. In one mode, the invention calculates acceleration fromDoppler velocity information and displays that color mapped accelerationof moving tissue. Combining the detected Doppler signals from movingtissue with a B-mode signal augments the B-mode image of moving tissueor, alternatively, gating the B-mode signal with the detected Dopplersignals from moving tissue may be used to block stationary tissue andclutter signals from the B-mode image.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is the block diagram of a typical color Doppler imaging systemuseful for color blood flow display;

FIG. 2 is a block diagram of the color Doppler tissue imaging system ofthis invention;

FIG. 3 illustrates alternative color and other look-up tables useful inthe invention illustrated in FIG. 2;

FIG. 4 illustrates a B-mode image of the heart and other tissue withsuperimposed clutter and noise;

FIG. 5 is a B-mode image of the heart with the moving myocardiumenhanced by the color Doppler means of this invention; and

FIG. 6 is a B-mode image of the heart with stationary tissue and clutterremoved by the Doppler gated system of this invention.

BRIEF DESCRIPTION OF PRIOR ART COLOR DOPPLER IMAGING SYSTEMS

FIG. 1 is the block diagram of a color Doppler imaging system similar tothat described in U.S. Pat. No. 5,014,710 entitled "Steered Linear ColorDoppler Imaging" issued May 14, 1991, which by reference is a part ofthis description. The transmitter 1 excites the acoustic transducer 2which propagates ultrasonic energy bursts into the body on a scan plane3. The transducer 2 usually comprises a phased array of acoustictransducer elements. Returning ultrasonic echoes are transduced intoelectrical signals that are amplified and focused by means ofreceiver/beamformer 4.

The received electrical signals are processed in one path by Dopplerimaging means including a multi-range gate Doppler detector 5, aprocessor that detects the Doppler shifted signals at multiple positionsalong each of a plurality of ultrasound scan lines. These signals arethen passed through a circuit designated fixed target canceller 6 thatremoves non-Doppler shifted signals corresponding to stationary clutter.Filters and thresholds are then applied at 7 for conditioning the signalto remove artifactual and clutter signals from the Doppler blood flowsignals. Then the Doppler frequencies are analyzed in a Dopplerfrequency analyzer 8 and a number of parameters (including mean,standard deviation and energy) are estimated for the multiple positionsalong the ultrasound scan line as is more particularly described in U.S.Pat. No. 5,014,710.

This analyzed Doppler information is stored for each line in colorDoppler scan converter 9 along with additional information from otherultrasound scan lines, comprising a complete two-dimensional colorDoppler scan frame. The scan converter 9 also translates or converts theDoppler information collected along the multiple ultrasound scan linesto a rectangular raster required for conventional video displays. Acolor is assigned to each position in the scan frame, according to thefrequency characteristics detected at that position by reference tocolor map look-up table 10. The color mapping considers mean frequency,standard deviation or energy, in combination or alone, as specified bythe user.

The video mixer 11 combines the color Doppler information with B-modeinformation acquired by B-mode imaging means in a second path includingthe B-mode detector 12 and scan converter 13. The video mixer presents acombined video signal for display to the video monitor 14.

BRIEF DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

FIG. 2 is the block diagram of the color Doppler tissue imaging systemof this invention including the B-mode and color Doppler imaging meansof FIG. 1. But, here, the fixed target canceller 6 and filters andthresholds 7 are removed from the system of FIG. 1. Alternate colorlook-up tables 20 are provided. An acceleration estimator 21 estimatesacceleration, for example, by subtracting data in the current velocityscan frame from the previous velocity scan frame in response to switchmeans 22. The color or grey-scale assigned to each position in the scanframe in a first mode of operation controlled by switch means 23 ismixed with B-mode information from B-mode scan converter 12 andpresented to the display monitor 14.

The switch means 22 also controls a second mode of operation that gatesB-mode information to the display 14 only when a color Doppler signal isdetected. A gate function 24 is shown schematically where the B-modesignal is gated from B-mode scan converter 13 and allowed through to thevideo display 14 only when a color Doppler signal is detected.

FIG. 3 shows alternate look-up tables. The first look-up table 30 showsa symmetric color distribution, thereby mapping colors based on themagnitude of the Doppler shift, rather than direction typically used incolor flow mapping. The second look-up table 31 shows a gray scaledistribution, which is useful when mixed with a conventional B-mode grayscale image to enhance or augment the myocardium as in FIG. 5. The thirdlook-up table 32 is made up entirely of black and is used along with thegate function 24. In this option, the B-mode pixels are combined in thevideo mixer with the corresponding black color Doppler pixels, therebyproducing the B-mode image with only the moving tissues displayed.

FIG. 4 shows a conventional B-mode echocardiogram where the myocardiumimage 40 is surrounded by other tissue and superimposed clutter andnoise 41. FIG. 5 shows a B-mode echocardiogram with the movingmyocardium 40 enhanced by this invention. FIG. 6 shows a B-modeechocardiogram where the stationary tissue and clutter has been removedby the gate function 24 of this invention to display only the myocardiumimage 40.

The clinician's scanning technique with this invention is similar toconventional B-mode image acquisition. When the appropriate scan planehas been achieved, the tissue motion detection mode can begin bypressing the appropriate key on the front panel. Moving tissuestructures then will be superimposed on the video display, along withthe conventional B-mode image.

The user can select a number of different operating conditions. Forexample, the user can choose to image either the velocities oraccelerations of the moving tissue by switch means 22. The user canselect either conventional color maps 10 to encode the direction as wellas the magnitudes of the velocities or accelerations, or use thesymmetric, non-directional color maps 20. The user can select agrey-scale "color" map 31 for the motion information and combine thiswith the B-mode information. Alternatively, the user can color theB-mode image and combine it with either a gray or color coded motionimage. The clinician can also remove the underlying B-mode information(by reducing the B-mode gain), and image only the color Dopplerinformation. Also, the user can select a "gated" B-mode operation byswitch means 23 where only the B-mode information from moving tissue isimaged as in FIG. 6. In this case, the B-mode image is displayed only ifa Doppler signal is detected at the corresponding location in the image.Alternatively, the B-mode could be gated by the absence of a Dopplersignal, thereby enhancing the boundary between the myocardium and thesurrounding tissue.

Stationary targets can be removed through the use of color maps, wherethe lowest velocities are assigned no color. Further, the range ofvelocities can be changed in the same way as with conventional colorDoppler imaging by adjusting the sampling rate of the Dopplerinformation.

In the strict sense, acceleration detection looks for differences in thevelocities from one time frame to the next. In the case where samevelocities are detected in the two adjacent time frames, there will beno acceleration measured. For moving tissue imaging, this loss of signalis distracting and counterintuitive. Therefore, for these operatingcases, the acceleration processor will assign a low value ofacceleration, instead of zero acceleration. The resulting image, whilenot truly displaying acceleration, has more diagnostic information. Inthe case where the velocities are zero in adjacent time frames, theacceleration is also assigned to zero.

Although this invention is described in the context of two-dimensionalDoppler processing methods, this processing is also applicable to colorM-mode operation. Tissue motion can be displayed in conjunction with aconventional M-mode display in similar ways as described fortwo-dimensional imaging.

Advantages of this invention over what has been done before include:

1. Slow moving targets corresponding to moving myocardium can bedetected with better sensitivity. Clinically, this additionalinformation assists in determining cardiac wall motion characteristics.

2. In some instances, acceleration mapping is more sensitive tomyocardial motion than velocity mapping. For example, atend-diastole/onset-of-systole the myocardium stops and reversesdirection. At this time the velocities are either zero or very low.However, the accelerations can be quite high at these times.Acceleration mapping presents motion information throughout more of thecardiac cycle, and generates images with greater clarity and sensitivitythan those available with velocity imaging.

3. With the gated B-mode operation, stationary noise and clutter can beremoved from the image.

4. The detection of endocardium is facilitated by a combination ofreducing the stationary clutter in the blood pools and enhancing themoving myocardium.

5. This invention facilitates the detection of the pericardium in thoseviews where the tissue adjacent to the myocardium is stationary.

6. This invention facilitates the detection of the myocardium wallthickness.

7. In the context of moving myocardium, magnitude color Doppler mappingis simpler to interpret than directional color Doppler mapping.

8. As the myocardium is augmented with this invention, both manual andautomatic detection of blood pool/myocardium borders is enhanced.

Although this method has been described on one type of color Dopplerimaging apparatus based on autocorrelation methods, it could be done onother systems, including other types of two-dimensional flow estimators.The symmetric color maps could be replaced by using conventional colormaps with non-directional Doppler detection. The accelerationcalculation can be done by means other than subtraction of adjacentvelocity frames, and still provide the same information.

Other variations in the invention as defined in the following claimswill be apparent to those skilled in the ultrasound imaging art.

We claim:
 1. An ultrasound tissue imaging system having an acoustictransducer and comprisingB-mode imaging means to produce with saidtransducer an electronically scanned B-mode image of tissue underexamination, said B-mode image substantially representing the intensityof echoes returned from said tissue along multiple B-mode scan lines;Doppler imagine means that accepts and processes large amplitude, lowfrequency signals to produce with said transducer an electronicallyscanned Doppler image of said moving tissue, the Doppler imagerepresenting estimates of velocity including means, standard deviationor energy derived from Doppler-shifted echoes reflected from said movingtissue at multiple sample volumes along multiple Doppler scan lines;color display means for displaying the B-mode image as a two-dimensionalimage with echo intensities encoded using a first mapping function andfor augmenting the B-mode image by simultaneously displaying saidestimates of velocity as a two-dimensional Doppler image using a secondand distinct mapping function that is spatially coordinated with andsuperimposed upon said b-mode image.
 2. The system of claim 1 furthercomprising means for estimating acceleration from said estimates ofvelocity and means for displaying estimates of that acceleration as thetwo-dimensional Doppler image using said second mapping function.
 3. Thesystem of claim 1 wherein the B-mode image is gated to the display bygate means only upon detection of said acquired and processed Dopplerinformation so as to remove stationary noise and clutter.
 4. The systemof claim 1 wherein the second mapping function color maps the magnitudeof forward and reverse motion without distinguishing direction of themotion.
 5. The system of claim 1 wherein the second mapping functiongrey-scale maps the Doppler information.
 6. A method of forming acomposite B-mode and color Doppler acoustic image of moving tissue bytransmitting acoustic pressure waves and receiving returned echoes onacoustic lines scanned by a transducer, said method comprising the stepsofderiving a B-mode image from echoes received along a first set ofimage scan lines, acquiring and processing color Doppler information atlarge amplitude and low frequencies from said moving tissue at multiplevolumes along scan lines that are the same as or propagatedindependently from the B-mode scan lines and displaying said colorDoppler information as a two-dimensional color coded image of movingtissue that is spatially coordinated with and superimposed upon saidB-mode image.
 7. The method of claim 6 wherein the step of acquiring andprocessing color Doppler information further includes deriving estimatesof acceleration for Doppler shifted echoes and displaying thoseestimates as the two-dimensional color coded image of said movingtissue.
 8. The method of claim 6 wherein the display of said B-modeimage is gated by said color Doppler information.
 9. An ultrasoundtissue imaging system having an acoustic transducer and comprisingB-modeimaging means to produce with said transducer an electronically scannedB-mode image of tissue under examination, said B-mode imagesubstantially representing the intensity of echoes returned from saidtissue along multiple B-mode scan lines; imaging means that accepts andprocesses large amplitude, low frequency signals to produce with saidtransducer an electronically scanned Doppler image of said movingtissue, the image representing estimates of velocity derived from echoesreflected from said moving tissue at multiple sample volumes alongmultiple scan lines; color display means for displaying the B-mode imageas a two-dimensional image with echo intensities encoded using a firstmapping function and for augmenting the B-mode image by simultaneouslydisplaying said estimates of velocity as a two-dimensional Doppler imageusing a second and distinct mapping function that is spatiallycoordinated with and superimposed upon said B-mode image.
 10. The systemof claim 9 further comprising means for estimating acceleration fromsaid estimates of velocity and means for displaying estimates of thatacceleration as the two-dimensional image using said second mappingfunction.
 11. The system of claim 10 wherein the second mapping functioncolor maps at a low non-zero value estimates of acceleration derivedfrom estimates of velocity when those velocities are the same but alsoare non-zero.
 12. The system of claim 9 wherein the bloodpool/myocardium borders are enhanced by gating the B-mode image to thedisplay means only in the presence of acquired and processed Dopplerinformation.
 13. A method of forming a color Doppler acoustic image ofmoving tissue by transmitting acoustic pressure waves and receivingreturned echoes on acoustic lines scanned by a transducer, said methodcomprising the steps ofacquiring and processing color Dopplerinformation at large amplitude and low frequencies from said movingtissue at multiple volumes along a plurality of scan lines anddisplaying said color Doppler information as a two-dimensional colorcoded image of said moving tissue.